Power conservation and latency minimization in frequency hopping communication networks

ABSTRACT

In one embodiment, a communication device samples a particular frequency hopping sequence during only a particular specified sub-timeslot of a timeslot. If a transmission energy is not detected during the specified sub-timeslot, the device turns off its receiver for a remainder of the timeslot. Otherwise, it continues to sample the particular frequency hopping sequence for at least one or more additional sub-timeslots of the remainder of the timeslot. In another embodiment, a communication device determines whether a neighboring communication device is operating in a first mode or a second mode. If in the second mode, it transmits a transmission to the neighboring communication device starting at any sub-timeslot of the plurality of sub-timeslots. If in the first mode, it transmits the transmission to the neighboring communication device while ensuring that the transmission is actively energized during a particular specified sub-timeslot.

TECHNICAL FIELD

The present disclosure relates generally to wireless communication, and,more particularly, to power consumption in frequency hopping wirelessnetworks.

BACKGROUND

Mesh networks are composed of two or more electronic devices eachcontaining at least one transceiver. The electronic devices use theirtransceivers to communicate with one another and/or a central device. Ifthe device wishes to communicate with another device that is out oftransmission range, the device may communicate via multi-hopcommunication through other devices. Because the devices may rely on asmall source of stored energy (e.g., batteries or a capacitor), it isdesirable for those devices to reduce power. In particular, thetransceiver, when placed in receive mode, can require significant powerand quickly drain a small source of stored energy. This is especiallyproblematic during a power outage of a main power supply, when only alimited lifespan of the backup power (e.g., battery or capacitor) may beused to transmit information.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments herein may be better understood by referring to thefollowing description in conjunction with the accompanying drawings inwhich like reference numerals indicate identically or functionallysimilar elements, of which:

FIG. 1 illustrates an example wireless network;

FIG. 2 illustrates an example wireless device/node;

FIG. 3 illustrates an example wireless message/packet;

FIGS. 4A-4C illustrate example frequency hopping sequences;

FIGS. 5A-7B illustrate examples of receiver operation;

FIGS. 8A-8B illustrate examples of transmitter operation;

FIG. 9 illustrates an example message exchange;

FIGS. 10A-10B illustrate an example simplified procedure for reducedpower consumption in frequency hopping computer networks, e.g., from theperspective of a node when receiving transmissions;

FIG. 11 illustrates another example simplified procedure for reducedpower consumption in frequency hopping computer networks, e.g., from theperspective of a node when transmitting;

FIG. 12 illustrates an example simplified procedure for node operationwhen the node loses power;

FIG. 13 illustrates an example simplified procedure for node operationwhen the node regains power; and

FIG. 14 illustrates an example simplified procedure for node operationwhen the node is powered.

DESCRIPTION OF EXAMPLE EMBODIMENTS Overview

According to one or more embodiments of the disclosure, a communicationdevice samples a particular frequency hopping sequence during only aparticular specified sub-timeslot of a timeslot. If a transmissionenergy is not detected during the specified sub-timeslot, the deviceturns off its receiver for a remainder of the timeslot. Otherwise, itcontinues to sample the particular frequency hopping sequence for atleast one or more additional sub-timeslots of the remainder of thetimeslot.

According to one or more additional embodiments of the disclosure, acommunication device determines whether a neighboring communicationdevice is operating in a first mode or a second mode. If in the secondmode, it transmits a transmission to the neighboring communicationdevice starting at any sub-timeslot of the plurality of sub-timeslots.If in the first mode, it transmits the transmission to the neighboringcommunication device while ensuring that the transmission is activelyenergized during a particular specified sub-timeslot.

DESCRIPTION

A computer network is a geographically distributed collection of nodesinterconnected by communication links and segments for transporting databetween end nodes, such as personal computers and workstations, or otherdevices, such as sensors, etc. Many types of networks are available,with the types ranging from local area networks (LANs) to wide areanetworks (WANs). LANs typically connect the nodes over dedicated privatecommunications links located in the same general physical location, suchas a building or campus. WANs, on the other hand, typically connectgeographically dispersed nodes over long-distance communications links,such as common carrier telephone lines, optical lightpaths, synchronousoptical networks (SONET), synchronous digital hierarchy (SDH) links, orPowerline Communications (PLC) such as IEEE 61334, IEEE P1901.2, andothers.

A wireless network, in particular, is a type of shared media networkwhere a plurality of nodes communicate over a wireless medium, such asusing radio frequency (RF) transmission through the air. For example, aMobile Ad-Hoc Network (MANET) is a kind of wireless ad-hoc network,which is generally considered a self-configuring network of mobileroutes (and associated hosts) connected by wireless links, the union ofwhich forms an arbitrary topology. For instance, Low power and LossyNetworks (LLNs), e.g., certain sensor networks, may be used in a myriadof applications such as for “Smart Grid” and “Smart Cities” (e.g., forAdvanced Metering Infrastructure or “AMI” applications) and may oftenconsist of wireless nodes in communication within a field area network(FAN). LLNs are generally considered a class of network in which boththe routers and their interconnect are constrained: LLN routerstypically operate with constraints, e.g., processing power, memory,and/or energy (battery), and their interconnects are characterized by,illustratively, high loss rates, low data rates, and/or instability.LLNs are comprised of anything from a few dozen and up to thousands oreven millions of LLN routers, and support point-to-point traffic(between devices inside the LLN), point-to-multipoint traffic (from acentral control point to a subset of devices inside the LLN) andmultipoint-to-point traffic (from devices inside the LLN towards acentral control point).

FIG. 1 is a schematic block diagram of an example (and vastlysimplified) computer network 100 (e.g., wireless or otherwise)illustratively comprising nodes/devices 200 (e.g., labeled as shown,“11,” “22,” “33,” and “44”) interconnected by frequency-hoppingcommunication links 105, as described below. In particular, certainnodes 200, such as, e.g., routers, sensors, computers, radios, etc., maybe in communication with other nodes 200, e.g., based on distance,signal strength, current operational status, location, etc. Thoseskilled in the art will understand that any number of nodes, devices,links, etc. may be used in the wireless network, and that the view shownherein is for simplicity (particularly, that while routers are shown,any wireless communication devices 11-44 may be utilized). Also, whilethe embodiments are illustratively shown herein with reference to agenerally wireless network, the description is herein is not so limited,and may be applied to networks that have wired links, wireless links,PLC links, etc.

Data transmissions 140 (e.g., traffic, packets, messages, etc. sentbetween the devices/nodes) may be exchanged among the nodes/devices ofthe computer network 100 using predefined network communicationprotocols such as certain known wireless protocols (e.g., IEEE Std.802.15.4, WiFi, Bluetooth®, etc.) or other shared media protocols whereappropriate (e.g., PLC). As described herein, the communication may bebased on a frequency-hopping protocol. In this context, a protocolconsists of a set of rules defining how the nodes interact with eachother.

FIG. 2 is a schematic block diagram of an example node/device 200 thatmay be used with one or more embodiments described herein, e.g., asnodes 11-44. The device may comprise one or more network interfaces 210(e.g., wireless/frequency-hopping), at least one processor 220, and amemory 240 interconnected by a system bus 250, as well as a main powersupply 260 (e.g., plug-in) and a backup power supply 265 (battery,capacitor, etc.).

The network interface(s) 210, e.g., transceivers, contain themechanical, electrical, and signaling circuitry for communicating dataover wireless links 105 coupled to the network 100. The networkinterfaces may be configured to transmit and/or receive data using avariety of different communication protocols as noted above and as willbe understood by those skilled in the art, particularly forfrequency-hopping communication as described herein. In addition, theinterfaces 210 may comprise an illustrative media access control (MAC)layer module 212 (and other layers, such as the physical or “PHY” layer,as will be understood by those skilled in the art). Note, further, thatthe nodes may have two different types of network connections 210,namely, wireless and wired/physical connections, and that the viewherein is merely for illustration.

The memory 240 comprises a plurality of storage locations that areaddressable by the processor 220 and the network interfaces 210 forstoring software programs and data structures associated with theembodiments described herein. Note that certain devices may have limitedmemory or no memory (e.g., no memory for storage other than for isprograms/processes operating on the device). The processor 220 maycomprise necessary elements or logic adapted to execute the softwareprograms and manipulate the data structures 245. An operating system242, portions of which are typically resident in memory 240 and executedby the processor, functionally organizes the device by, inter alia,invoking operations in support of software processes and/or servicesexecuting on the device. These software processes and/or services maycomprise routing process/services 244, and an illustrative “modeselection” process 248 as described in greater detail below. Note thatwhile mode selection process 248 is shown in centralized memory 240,alternative embodiments provide for the mode selection process to bespecifically operated within the network interfaces 210, such as acomponent of MAC layer 212 (process “248 a”).

It will be apparent to those skilled in the art that other processor andmemory types, including various computer-readable media, may be used tostore and execute program instructions pertaining to the techniquesdescribed herein. Also, while the description illustrates variousprocesses, it is expressly contemplated that various processes may beembodied as modules configured to operate in accordance with thetechniques herein (e.g., according to the functionality of a similarprocess). Further, while the processes have been shown separately, thoseskilled in the art will appreciate that processes may be routines ormodules within other processes.

Routing process (services) 244 contains computer executable instructionsexecuted by the processor 220 to perform functions provided by one ormore routing protocols, such as proactive or reactive routing protocolsas will be understood by those skilled in the art. These functions may,on capable devices, be configured to manage a routing/forwarding table(e.g., a data structure 245) containing, e.g., data used to makerouting/forwarding decisions. In particular, in proactive routing,connectivity is discovered and known prior to computing routes to anydestination in the network, e.g., link state routing such as OpenShortest Path First (OSPF), orIntermediate-System-to-Intermediate-System (ISIS), or Optimized LinkState Routing (OLSR). Reactive routing, on the other hand, discoversneighbors (i.e., does not have an a priori knowledge of networktopology), and in response to a needed route to a destination, sends aroute is request into the network to determine which neighboring nodemay be used to reach the desired destination. Example reactive routingprotocols may comprise Ad-hoc On-demand Distance Vector (AODV), DynamicSource Routing (DSR), DYnamic MANET On-demand Routing (DYMO), etc.Notably, on devices not capable or configured to store routing entries,routing process 244 may consist solely of providing mechanisms necessaryfor source routing techniques. That is, for source routing, otherdevices in the network can tell the less capable devices exactly whereto send the packets, and the less capable devices simply forward thepackets as directed.

Notably, mesh networks have become increasingly popular and practical inrecent years. In particular, shared-media mesh networks, such aswireless or PLC networks, etc., are often on what is referred to asLow-Power and Lossy Networks (LLNs), which are a class of network inwhich both the routers and their interconnect are constrained: LLNrouters typically operate with constraints, e.g., processing power,memory, and/or energy (battery), and their interconnects arecharacterized by, illustratively, high loss rates, low data rates,and/or instability. LLNs are comprised of anything from a few dozen andup to thousands or even millions of LLN routers, and supportpoint-to-point traffic (between devices inside the LLN),point-to-multipoint traffic (from a central control point such at theroot node to a subset of devices inside the LLN) and multipoint-to-pointtraffic (from devices inside the LLN towards a central control point).

An example protocol specified in an Internet Engineering Task Force(IETF) Internet Draft, entitled “RPL: IPv6 Routing Protocol for LowPower and Lossy Networks”<draft-ietf-roll-rpl-19> by Winter, at al.(Mar. 13, 2011 version), provides a mechanism that supportsmultipoint-to-point (MP2P) traffic from devices inside the LLN towards acentral control point (e.g., LLN Border Routers (LBRs) or “rootnodes/devices” generally), as well as point-to-multipoint (P2MP) trafficfrom the central control point to the devices inside the LLN (and alsopoint-to-point, or “P2P” traffic). RPL (pronounced “ripple”) maygenerally be described as a distance vector routing protocol that buildsa Directed Acyclic Graph (DAG) for use in routing traffic/packets 140,in addition to defining a set of features to bound the control traffic,support repair, etc. Notably, as may be appreciated by those skilled inthe art, RPL also supports the concept of Multi-Topology-Routing (MTR),whereby multiple DAGs can be built to carry traffic according toindividual requirements.

FIG. 3 illustrates an example simplified message/packet format 300 thatmay be used to communicate information between devices 200 in thenetwork. For example, message 300 illustratively comprises a header 310with one or more fields such as a source address 312, a destinationaddress 314, a length field 316, a type field 318, as well as otherfields, such as Cyclic Redundancy Check (CRC) error-detecting code toensure that the header information has been received uncorrupted, aswill be appreciated by those skilled in the art. Within the body/payload320 of the message may be any information to be transmitted, such asuser data, control-plane data, etc. In addition, based on certainwireless communication protocols, a preamble 305 may precede the message300 in order to allow receiving devices to acquire the transmittedmessage, and synchronize to it, accordingly.

Frequency-hopping, also referred to as “frequency-hopping spreadspectrum” (FHSS) or channel-hopping, is a method of transmitting radiosignals by rapidly switching a carrier among numerous frequencychannels, e.g., using a pseudorandom sequence known to both transmitterand receiver. For example, frequency-hopping may be utilized as amultiple access method in the frequency-hopping code division multipleaccess (FH-CDMA) scheme. Generally, as may be appreciated by thoseskilled in the art, transmission using frequency-hopping is differentfrom a fixed-frequency transmission in that frequency-hoppedtransmissions are resistant to interference and are difficult tointercept. Accordingly, frequency-hopping transmission is a usefultechnique for many applications, such as sensor networks, LLNs, militaryapplications, etc.

In particular, as shown in FIG. 4A, in frequency-hopping wirelessnetworks, time frames are divided within a frequency-hopping sequence400 into regular timeslots 410, each one operating on a differentfrequency 430 (e.g., f₁-f₄). A reference clock may be provided for thetime frames for an entire network (e.g., mesh/cell), or at least betweenpairs of communicating devices. A MAC layer 212 of each node 200 dividestime into timeslots that are aligned with the timeslot boundary of itsneighbor. Also, each timeslot 410 may be further divided intosub-timeslots 420. (Note that not all frequency-hopping systems usesub-timeslots, and devices can begin transmission at any time within atimeslot; the view herein is merely one example.) Illustratively, theMAC layer 212 is in charge of scheduling the timeslot in which a packetis sent, the main objective of which generally being randomization ofthe transmission time in order to avoid collisions with neighbors'packets. Note that the MAC layer 212 must not only schedule the datamessages coming from upper layers of a protocol stack, but it also mustschedule its own packets (e.g., acknowledgements, requests, beacons,etc.).

A device in the frequency-hopping network configures its receiver tofollow a hopping schedule by picking a channel sequence, duration ofeach time slot, and time base that defines when the first slot in theschedule begins. To then communicate a packet, the transmitter andreceiver must be configured to the same channel during the packettransmission. All devices in a given network may utilize the samehopping schedule (i.e. all devices are configured to use the samechannel sequence, time slot duration, and a common time base), resultingin a network where all communication in the network at any given pointin time utilizes the same channel. An example of this is shown in FIG.4B, in which each receiver (22, 33, and 44) are all configured with thesame sequence (assume also that node 11 uses the same sequence).

Alternatively, each transmitter-receiver pair may utilize differenthopping schedules (i.e., each pair may differ in channel sequence, timeslot duration, and/or time base), such that transmitter-receiver pairsmay communicate at the same time but on different channels. Forinstance, each device in the channel hopping network may individuallypick their own hopping schedule parameters independent of any othernode, as is shown in FIG. 4C. Note that the offset of the frequencies(i.e., the fact that the same four frequencies are used in the sameorder, just offset by one timeslot) is merely one illustration, and thesequences and frequencies can be independently chosen. Also, note thatwhile timeslots are shown as being synchronized between different nodes,those skilled in the art will appreciate that timeslots betweendifferent nodes can, in fact, be out-of-phase, and may have norelationship with each other.

A device synchronizes its hopping schedule with another device by iscommunicating its channel sequence, time slot duration, and current timewithin the hopping schedule. Hopping schedule parameters may becommunicated in explicit synchronization packets and/or piggybacked onexisting data packets. As mentioned, some of these parameters (e.g.,channel sequence) may be network-wide and implicit. Devices store theseparameters to know what channel to use for transmission at a particulartime.

As noted above, because devices in a computer network (e.g., meshnetwork) may rely on a small source of stored energy (e.g., batteries ora capacitor), it is desirable for those devices to reduce power. Inparticular, the transceiver, when placed in receive mode, can requiresignificant power and quickly drain a small source of stored energy.This is especially problematic during a power outage of a main powersupply, when only a limited lifespan of the backup power (e.g., batteryor capacitor) may be used to transmit and/or receive information. Theremay be other reasons where reduced power is desired, such as in general(all the time), in response to a demand response from a power grid(e.g., to prevent brownouts, blackouts, etc.), or other reasons.

Power Reduction and Latency Minimizing Modes

The techniques described herein provide a system and method that canallow devices to dynamically transition between a mode that minimizespower (by disabling the receiver whenever possible) and a mode thatminimizes latency (by enabling the receiver whenever possible) withlittle overhead. The system and method permits a limited increase inexpected communication latency when transitioning to a mode thatminimizes power. In particular, supporting both latency-minimizing andpower-reduction modes within the same network allows communicationbetween devices with different power requirements in the same network,and adapts to possible changes (expected or unexpected) in a powersource for certain devices.

Specifically, according to one or more embodiments of the disclosure asdescribed in greater detail below, in a power reduction mode, acommunication device samples a particular frequency hopping sequenceduring only a particular specified sub-timeslot of a timeslot. If atransmission energy is not detected during the specified sub-timeslot,the is device turns off its receiver for a remainder of the timeslot.Otherwise, it continues to sample the particular frequency hoppingsequence for at least one or more additional sub-timeslots of theremainder of the timeslot. According to one or more additionalembodiments of the disclosure, a communication device determines whethera neighboring communication device is operating in a first mode (powerreduction) or a second mode (latency minimizing). If in the second mode,it transmits a transmission to the neighboring communication devicestarting at any sub-timeslot of the plurality of sub-timeslots. If inthe first mode, it transmits the transmission to the neighboringcommunication device while ensuring that the transmission is activelyenergized during a particular specified sub-timeslot.

Illustratively, the techniques described herein may be performed byhardware, software, and/or firmware, such as in accordance with modeselection process 248 and/or MAC layer module 212 (248 a), which mayeach contain computer executable instructions executed by a processor(e.g., processor 220 or an independent processor within the networkinterface 210) to perform functions relating to the novel techniquesdescribed herein, such as, e.g., as part of a frequency hoppingcommunication protocol. For example, the techniques herein may betreated as extensions to conventional wireless communication protocols,such as the 802.11 protocol, WiFi, etc., and as such, would be processedby similar components understood in the art that execute such protocols,accordingly.

Operationally, a communication device (e.g., nodes 11-44) may selectbetween operation in a first “latency-minimizing” mode and a second“power-reduction” mode, illustratively depending upon various triggersor configurations as described herein. To minimize communication latencyin the latency-minimizing mode, the devices generally always enabletheir receivers configured to the channel specified by the hoppingschedule, except when switching channels or transmitting packets. Inother words, the receivers sample the particular frequency hoppingsequence 400 during all non-transmitting sub-timeslots 420. A device canthus initiate transmissions to a latency-minimizing device at any time,since the receiver is enabled nearly all the time. Accordingly, suchtransmissions may utilize the shortest preamble allowed by the physicallayer.

Conversely, to reduce power, the devices utilize a sampling techniquethat is applied to the same hopping schedule 400 as in thelatency-minimizing mode. Specifically, each sample occurs during only aparticular specified sub-timeslot 420, e.g., at the beginning of atimeslot 410, using the channel/frequency assigned to the timeslot. Whenno transmission energy is detected during the specified sub-timeslot,the device disables (turns off) the receiver for the remainder of thetimeslot.

FIGS. 5A and 5B illustrate the differences between thelatency-minimizing mode (FIG. 5A) and power-reduction mode (FIG. 5B),where the graphs demonstrate example receiver-enabled timingcorresponding to each mode. As can be seen, the receiver is enablednearly all the time during latency-minimizing mode (unlesstransmitting), while in power-reduction mode, the receiver is onlyenabled for a very small percentage of the frequency hopping sequence.FIG. 5B, notably, illustrates the instance where no transmission energyis detected during the sampling periods.

If there is transmission energy detected during a sample, then thereceiving device continues to sample the channel for at least one ormore sub-timeslots of the remainder of the timeslot. In particular, thereceiver may remain active long enough to determine whether thetransmission energy is a transmission (message/packet 140) meant for thecommunication device. For example, FIG. 6A illustrates the occurrence ofa transmission energy 660, and the receiver remains enabled. From thereceiving node's perspective, when a header 310 of a packet 300 isreceived, notably before the whole packet needs to have been received,the node (e.g., its MAC layer 212) analyzes the destination address 314or other indication to determine whether the receiving node is theintended recipient of the packet 300. Illustratively, this analysis mayoccur after performing an error check on the header of the packet toensure that the information is error free.

As shown in FIG. 6A, in response to verifying that the receiving nodeis, in fact, the intended recipient node (e.g., that the destinationaddress corresponds to the wireless node), then the receiver may remainenabled (be kept on) for the duration of the transmission (e.g., basedon length field 314, or for the duration of a standard packet, or forthe remainder of the timeslot, etc.). Otherwise, if it is determinedthat the destination address corresponds to another wireless node, i.e.,that the transmission is not meant for the communication device, thenthe receiver may turn off and ignore the remainder of the packet (e.g.,payload 320), as shown in FIG. 6B.

By using the channel sampling technique, a device can achieve a smallreceiver duty cycle while only increasing expected latency to T/2, whereT is the timeslot duration. For example, a typical system availabletoday can perform a channel sample in less than 400 us, thus achievingabout 0.3% duty cycle and 62.5 ms expected latency with a time slotduration of 125 ms. Further duty cycle reduction (and expected latencyincrease for communication) is possible by disallowing communicationduring a subset of slots.

Note that according to the techniques herein the channel sequence andtiming are still known and active, i.e., the network stack of anode/device remains “awake” (unlike sleep timers). That is, using thesame hopping schedule 400 in both modes allows devices to transitionbetween latency-minimizing and power-reduction mode while allowingchannel schedule information synchronized with other devices to remainvalid, minimizing the complexity and cost of transitioning betweenmodes.

While the sampling technique is shown based on a single sub-timeslot 420in each timeslot (e.g., an initial sub-timeslot of each timeslot), thetechniques herein may also utilize other arrangements. For example, asshown in FIG. 7A, a particular sub-timeslot of only certain timeslots(e.g., every other timeslot) of the particular frequency hoppingsequence may be used to further reduce power consumption. In thisinstance, certain provisions may be made to ensure that communicatingdevices knew which particular timeslots carried the specifiedsub-timeslots for active power-reduction communication. Also, as shownin FIG. 7B, the number of sub-timeslots specified for receiver activitymay be increased, thus lessening the power reduction, but alsodecreasing the associated latency. As shown in FIG. 7B, for example, twosub-timeslots are used during each timeslot, accordingly.

A device may quickly and deterministically switch to power-reductionmode, and vice versa, and for each switch maintains the same frequencyhopping sequence, only changing the sampling rate. Note that a devicemay proactively notify/inform other neighboring devices of its currentmode, such as by using a synchronization message or piggybacking suchinformation in other outgoing data packets. Alternatively (or inaddition), a device may reactively notify other devices of its currentmode by sending such information only in response to receiving a packetfrom those devices (e.g., in an acknowledgement message having theindication piggybacked therein). This reactive mode is also useful fornotifying those devices that did not receive any proactive announcementsin the case one or more were sent. As a still further alternative, nonotification may be sent, and any devices wishing to transmit a messageto the receiving device may be forced to make a determinationintrinsically, such as described below.

In accordance with one or more embodiments herein, a device may transmitmessages toward another device in the network 100 based on whichevermode the intended recipient device is currently operating. Inparticular, a transmitting device may determine that a neighboringcommunication device is operating in the power-reduction mode or thelatency-minimizing mode in response to receiving an explicit indicationwithin a message received from the neighboring communication device thatinforms the communication device of the mode in which the neighboringcommunication device is operating.

In response to determining that the neighboring communication device isoperating in the latency-minimizing mode, transmissions may be sent(transmitted) to the neighboring communication device starting at anysub-timeslot 420 of a timeslot 410, as illustrated in FIG. 8A.Conversely, in response to determining that the neighboringcommunication device is operating in the power-reduction mode, thetransmission to the neighboring communication device must ensure that itis actively energized during the particular specified sub-timeslot(e.g., an initial sub-timeslot), as illustrated in FIG. 8B. Forinstance, as shown in FIG. 8B, one or more “wake-up messages” 880 mayprecede the actual transmission 660 in order to account for any possibleclock-drift of the intended receiver. Specifically, a device caninitiate transmissions to a power-reducing device only during thespecified sub-timeslot 420, and the transmission generally consists of ashort preamble (e.g., wake-up messages 880 and/or packet preamble 305)followed by the data packet 300/140 (transmission 660). Because time issynchronized, the preamble is sized to account only for the expectederror in time synchronization (e.g., clock-drift) between thetransmitter and receiver.

Note that a device can transition from the power-reduction mode tolatency-minimizing mode at any time. Packet transmissions fromtransmitting devices unaware of the transition will still be received,however, since the receiver is still enabled at the same times andchannels as in the power-reduction mode. That is, if the transmission issent such that it is received in the specified sub-timeslot of thepower-reduction mode, then if the receiver is in the latency-minimizingmode, the receiver is still active during this sub-timeslot, and thetransmission may still be received. However, a device can alsotransition from the latency-minimizing mode to power-reduction mode atany time. In this instance, unless an explicit notification is sent outby the receiving node, messages may go unheard by the receiving node ifnot sent during the specified sub-timeslot. In other words, packettransmissions from other devices unaware of the transition will not bereceived unless they happen to be sent during the specified sub-timeslot(e.g., at the beginning of a timeslot) where a channel sample occurs.

In accordance with one or more embodiments herein, therefore, though theneighboring communication device (receiving node) can be assumed to bein the latency-minimizing mode as a default (“normal”) mode, if there isa lack of acknowledgement 945 received from the neighboringcommunication device for a particular communication (message 140/660),as shown in FIG. 9, then the transmitting device may attempt subsequenttransmissions under the assumption that the receiving device hastransitioned to operate in a power-reduction mode. (Alternatively, aplurality of unacknowledged transmissions may be tolerated beforeassuming the intended receiver is in power-reduction mode.) If, afterfurther transmissions in the power-reduction mode, it is determined thatacknowledgements are still not received, then the device may no longeris be reachable, and may thus be declared unreachable, accordingly.

It is important to note that the techniques herein do not merely reducepower by having receivers listening to only a portion of a timeslot 410.Indeed, the techniques herein have transmitters precede datatransmissions with wakeup packet transmissions, and as a result, doesnot require the listener (receiving device) to remain active for aduration that accounts for any clock drift or time synchronizationerror. That is, because the techniques herein uses wakeup packets, itshifts the cost of handling clock drift/sync errors to the transmitter.The assumption is that transmissions occur less frequently (there willinevitably be times where a receiver wakes up to listen and there isnothing to receive) and this is a particularly useful tradeoff. Thereceiver (radio) may thus be enabled just long enough to sample energyand immediately be turned off if no energy is detected.

Furthermore, the techniques herein make no assumption that transmissionslots never overlap between devices in close proximity in order to avoidcollisions. In particular, the techniques herein allow for the use ofmechanisms to resolve contention between multiple transmitters byrandomizing the number of wakeup packets to transmit before a datapacket. In this manner, the transmitters can effectively choose a randomtransmission start time to and utilize standard CSMA/CA (carriersensing) techniques help avoid collisions.

FIGS. 10A-10B illustrate an example simplified procedure for reducedpower consumption in frequency hopping computer networks in accordancewith one or more embodiments described herein, e.g., from theperspective of a node when receiving transmissions. The procedure 1000starts at step 1005, and continues to step 1010, where the node isoperating according to particular frequency hopping sequence 400 ingeneral, as described above. Upon determining in step 1015 that the nodeis in the second mode (normal mode or latency-minimizing mode), then instep 1020 the node samples the frequency hopping sequence during allsub-timeslots 420 of the particular frequency hopping sequence 400.(Note that in step 1015, the node may also be configured toinform/notify its neighbors as to its current mode of operation.)

If, on the other hand, the node in step 1015 is operating in the secondmode (power reduction or power-outage mode), then in step 1025 the nodeconfigures itself (e.g., MAC layer 212) to sample the particularfrequency hopping sequence 400 during only a specified sub-timeslot 420of a timeslot 410, such as, e.g., an initial sub-timeslot, periodictimeslots, etc., as noted above. If in step 1030 there is transmissionenergy 660 detected in sampled sub-timeslot, then the device continuesto sample the particular frequency hopping sequence in step 1035 for atleast one or more sub-timeslots of the remainder of the timeslot 410,e.g., long enough to determine in step 1040 whether the transmission 660was meant for the node/device. If there is no energy in step 1030, or ifthe transmission is not meant for the node in step 1040, then thereceiver 210 is turned off in step 1045, until the next sample period.Conversely, if the transmission is intended for the receiving node instep 1040, then the receiver is kept on in step 1050 for at least aduration of the transmission (e.g., based on the length of a packet orother known or determinable length of time).

In addition, FIG. 11 illustrates another example simplified procedurefor reduced power consumption in frequency hopping computer networks inaccordance with one or more embodiments described herein, e.g., from theperspective of a node when transmitting transmissions. The procedure1100 starts at step 1105, and continues to step 1110, where the nodedetermines whether a neighboring communication device is operating in afirst mode (power-reduction mode) or a second mode (latency minimizationor normal mode). If in the second mode in step 1115 (e.g., an assumeddefault mode), then in step 1120 the node may transmit a transmission660 (e.g., packet 140) to a neighboring communication device starting atany sub-timeslot 420 of the frequency hopping sequence 400. If there isno acknowledgment (“ACK”) in step 1125 (or given number of ACKs), thenthe node may determine or assume that the neighboring communicationdevice is in the first mode (power-reduction mode) in step 1130.

If it is determined/assumed that the neighboring communication device isin the first mode of operation in step 1130, e.g., based on anindication from neighboring is communication device or based on no ACKmessages in step 1125, then any transmissions may be transmitted in step1135 to the neighboring communication device while ensuring that thetransmission is actively energized during a particular specifiedsub-timeslot 420, e.g., the first sub-timeslot. For instance, asdescribed above, a series of wake-up messages 880 may precede the actualtransmission 660, such as to account for any clock drifting by theneighboring device. If there is still no ACK (or given number of ACKs)in step 1140 for the transmission in the power-reduction specifiedsub-timeslot, then in step 1145 the neighboring communication device maybe declared unreachable.

The procedure 1100 ends in step 1150, notably with the ability to returnto continue transmitting further transmissions/packets, or else to makeupdated determinations as to whether a particular neighboringcommunication device is in the first or second mode of operation.

It should be noted that while certain steps within procedures 1000-1100may be optional as described above, the steps shown in FIGS. 10-11 aremerely examples for illustration, and certain other steps may beincluded or excluded as desired. Further, while a particular order ofthe steps is shown, this ordering is merely illustrative, and anysuitable arrangement of the steps may be utilized without departing fromthe scope of the embodiments herein. Moreover, while procedures1000-1100 are described separately, certain steps from each proceduremay be incorporated into each other procedure, and the procedures arenot meant to be mutually exclusive.

The novel techniques described herein, therefore, provide for differentreceive modes in a frequency hopping network. By allowing a receiver tosample only select portions of a frequency hopping sequence, devices mayenter into a power-conservative mode, whether in response to a loss ofprimary power, or simply to conserve energy resources, generally, asdescribed above. In addition, the techniques herein maintain a reducedpreamble length during the power-reduction mode by utilizing the sametime synchronization used in the frequency hopping sequence, and alsoalleviate costly resynchronization of the hopping schedule with otherdevices by using the same hopping schedule in both modes.

In addition, the techniques herein provide an efficient balance betweenpower conservation and communication latency by switching between modesof operation. In particular, by merely disallowing communication duringa subset of slots in the hopping schedule is problematic because doingso also greatly increases the expected communication latency. Forinstance, if the system disallows communication in all but onesub-timeslot in a period with N sub-timeslots, while the system would beable to achieve an average duty-cycle of 1/N, the expected communicationlatency would be (N−1)*T/2, where T is the timeslot duration. Forexample, achieving a 1% duty-cycle would increase the expected latencyto nearly 50 timeslots. Using the techniques herein, therefore, reducethe duty-cycle at certain times (e.g., when power reduction is desiredor necessary), and reduce the latency at other times.

For example, as mentioned briefly above, the techniques herein areparticularly useful in response to power outages, such as to ensure thatthe power reduction allows for as many (often critical) messages aspossible to be transmitted and/or received before backup power iscompletely lost. Specifically, for Smart Grid AMI applications, handlingpower outage scenarios reliably and effectively is a challengingproblem. Existing mesh-networking solutions that support AMIapplications provide some capability to report power outage events. Forexample, devices may broadcast a Power Outage Notification (PON) or“Last Gasp” message in hopes of communicating the event to a neighboringdevice that is still powered.

Using the proper low-power hardware/software designs, such as thatdescribed above, it is possible to extend the lifetime of a meter fromminutes to hours. For example, the above mechanisms allow a device toprovide networking functions even while reducing its listeningduty-cycle to 1% or less by trading extended lifetime for reducedchannel capacity. However, the ability to maintain a network providesthe following advantages:

-   -   1) Power Outage Notifications can utilize unicast communication,        making them more efficient and reliable than when using        broadcast communication;    -   2) Devices experiencing a power outage can receive and forward        Power is Outage Notifications from devices that are not within        transmission range of a powered device;    -   3) Network restoration messages can be communicated with lower        latency, since devices continue to maintain link-layer        synchronization with neighbors and a routing topology; and    -   4) The network remains connected, whereas it may become        disconnected if devices experiencing a power outage stop        networking functions. In addition to providing additional        information of exactly which devices are experiencing a power        outage, network restoration notifications can be communicated        even when power to critical parts of the network has not been        restored.

The techniques described above specifically allow a device to switchbetween a mode that is fully capable and maximizes channel capacity to amode that reduces power consumption with reduced channel capacity.Below, an illustrative example of how a device uses the mechanisms tosupport a power outage scenario is described, i.e., a method that allowsportions of the network affected by a power outage event to switch intoa power-reduced mode and continue to maintain the network.

FIG. 12 illustrates an example simplified procedure for node operationwhen the node loses power. The procedure 1200 starts at step 1205, andcontinues to step 1210, where a device 200 experiences a power outage,and in step 1215 immediately switches into a power-reduced mode asdescribed above to perform the following functions:

-   -   Step 1220—Begin listening for transmissions using        channel-sampling techniques. In doing so, the device        significantly reduces the power required to listen for and        receive messages.    -   Step 1225—Optionally set a flag for each neighbor in a neighbor        table to indicate that transmissions should assume the        reduced-power mode. This conservative behavior assumes that        power outage events are spatially correlated and minimizes        wasted energy when erroneously assuming that a neighbor is in        fully capable mode. Note that a device in fully capable mode        will still receive is messages sent to it using power-reduced        mode. When receiving a positive indication from a neighbor that        it is in fully capable mode, the device can reflect that state        in the neighbor table. (The indication may be included in any        link data or acknowledgment packet transmitted by that        neighbor.)    -   Step 1230—Transmit a unicast Power Outage Notification (PON)        message to the head-end server. Because all devices experiencing        a power outage can continue to receive and forward messages,        devices can continue to route messages as they did before.    -   Step 1235—Classify received messages and drop messages that are        not allowed during a power outage event. Because the network is        operating with reduced channel capacity, some traffic may be        dropped to conserve energy or channel capacity. The classes may        utilize the DSCP field in an IP packet. (Class assignment is        application specific.)

The procedure 1200 then illustratively ends in step 1240, such as inresponse to regaining power, or in response to losing all backup powerand operational capabilities.

For example, FIG. 13 illustrates an example simplified procedure fornode operation when the node regains power. The procedure 1300 starts atstep 1305, and continues to step 1310, where the node experiences apower restoration, and immediately switches into a fully capable mode instep 1315 to performs the following functions:

-   -   Step 1320—Begin listening all the time (i.e., stop using        channel-sampling techniques) to increase the channel capacity.    -   Step 1325—Transmit a unicast Power Restoration Notification        message to the head-end server. Because all devices experiencing        a power outage can continue to receive and forward messages,        devices can continue to route messages as they did before.    -   Step 1330—The node then begins to behave as a powered device,        which is described below with reference to FIG. 14. Note that        the node in the illustrative embodiment does not make any        changes to the neighbor table due to a power restoration.        Instead, it relies on positive indication that a neighbor has        restored power.

The procedure 1300 then ends in step 1335, where the node has restoredpower, and proceeds to operate as a powered node according to procedure1400 of FIG. 14.

Specifically, FIG. 14 illustrates an example simplified procedure fornode operation when the node is powered. The procedure 1400 starts atstep 1405, and continues to step 1410, where the node becomesoperational in a powered state. Note that nodes/devices that areunaffected by the power outage must receive explicit notification fromdevices that have switched into a reduced-power mode. The behaviors ofpowered devices that support the reduced-power mode are as follows:

-   -   Step 1415—Determine that a neighboring device (next-hop        destination) has transitioned into reduced-power mode: 1) when        the number of consecutive unicast transmissions that fail to        receive an acknowledgment exceeds a threshold, or 2) when        receiving the first positive indication that a device has        transitioned into a reduced-power mode. Note that a flag may be        set for each neighbor in the neighbor table to indicate that        transmissions should assume the reduced-power mode. This        conservative behavior assumes that power outage events are        spatially correlated and ensures that it can transmit messages        to a power outage device. A device in reduced-power mode will        include a flag in its link frames to indicate that it is        operating in reduced-power mode. When receiving a positive        indication from a neighbor that it is in fully capable mode, the        device can then reflect that state in the neighbor table. (The        indication may be included in any link data or acknowledgment        packet transmitted by that neighbor.)    -   Step 1420—When forwarding a datagram, check if the next-hop        destination is operating in reduced-power mode. The        determination result reflected in step 1425.    -   Step 1430—Perform normal forwarding when the next-hop is not in        power is reduction mode.    -   Step 1435—If the next-hop is in power reduction, first determine        whether the packet is in a class not allowed for forwarding        during a power outage event (e.g., based on type field 318, such        as whether the message is a PON, or other critical data).    -   Step 1440—If not allowed in step 1435, then drop the packet to        avoid wasting energy and channel capacity for the reduced-power        device.    -   Step 1445—If allowed in step 1435, then forward the packet        according to the reduced power mode.

The procedure 1400 ends in step 1450, notably with the ability to returnto an appropriate step above, such as when new packets are received forforwarding, or else when new notifications are received, etc.

Again, it should be noted that while certain steps within procedures1200-1400 may be optional as described above, the steps shown in FIGS.12-14 are merely examples for illustration, and certain other steps maybe included or excluded as desired. Further, while a particular order ofthe steps is shown, this ordering is merely illustrative, and anysuitable arrangement of the steps may be utilized without departing fromthe scope of the embodiments herein. Moreover, while procedures1200-1400 are described separately, certain steps from each proceduremay be incorporated into each other procedure, and the procedures arenot meant to be mutually exclusive.

A system in accordance with this illustrative embodiment, therefore,continues to operate the network in a reduced-power mode during a poweroutage event. Specifically, this is achieved by: 1) a power-outagedevice using channel-sampling techniques to reduce power and increaselifetime while listening and receiving messages, 2) a power-outagedevice assuming that its neighbors have switched into a reduced-powermode due to spatial locality of power outage events, 3) a powered deviceassuming that its neighbors have switched into a reduced-power modeafter receiving positive indication that at least one neighbor hasswitched into a reduced-power mode due to spatial locality is of poweroutage events, 4) a power-outage device dropping all received messagesthat are not allowed for forwarding during a power outage event toreduce power and channel utilization, and 5) a powered device droppingcertain messages that have a next-hop destination to a reduced-powerdevice to reduce power and channel utilization.

A system in accordance with this illustrative embodiment: 1) increasesthe lifetime of a network during a power outage event by utilizingchannel-sampling techniques and dropping unimportant traffic, 2) allowsdevices that have no powered neighbors to communicate Power OutageNotification messages and any other important message by utilizingpower-outage devices operating in reduced power mode, 3) decreaseslatency of network restoration by continuing to maintain channel-hoppingsynchronization and the routing topology, and 4) supports otherdiagnostic protocols that would otherwise not be possible if devicescould no longer communicate across the network.

While there have been shown and described illustrative embodiments thatprovide for different receive modes in a frequency hopping network, itis to be understood that various other adaptations and modifications maybe made within the spirit and scope of the embodiments herein. Forexample, the embodiments have been shown and described herein withrelation to wireless networks, such as LLNs. However, the embodiments intheir broader sense are not as limited, and may, in fact, be used withother types of networks and/or protocols, such as PLC (power linecommunication). Also, while the description above relates to packets andpacket headers, the techniques may be equally applicable tonon-packetized transmissions.

The foregoing description has been directed to specific embodiments. Itwill be apparent, however, that other variations and modifications maybe made to the described embodiments, with the attainment of some or allof their advantages. For instance, it is expressly contemplated that thecomponents and/or elements described herein can be implemented assoftware being stored on a tangible (non-transitory) computer-readablemedium (e.g., disks/CDs/etc.) having program instructions executing on acomputer, hardware, firmware, or a combination thereof. Accordingly thisdescription is to be taken is only by way of example and not tootherwise limit the scope of the embodiments herein. Therefore, it isthe object of the appended claims to cover all such variations andmodifications as come within the true spirit and scope of theembodiments herein.

1. A method, comprising: operating a communication device according to aparticular frequency hopping sequence in a communication network, theparticular frequency hopping sequence having a plurality of timeslotseach corresponding to a particular frequency within the frequencyhopping sequence, each timeslot divided into a plurality ofsub-timeslots; and operating the communication device in a first mode,wherein the first mode comprises: sampling the particular frequencyhopping sequence by a receiver of the communication device during only aparticular specified sub-timeslot of a timeslot; detecting whether atransmission energy is detected during the specified sub-timeslot; inresponse to detecting that there is no transmission energy during thespecified sub-timeslot, turning off the receiver for a remainder of thetimeslot; and is in response to detecting that there is transmissionenergy during the specified sub-timeslot, continuing to sample theparticular frequency hopping sequence for at least one or moreadditional sub-timeslots of the remainder of the timeslot.
 2. The methodas in claim 1, wherein sampling during only the specified sub-timeslotcomprises sampling an initial sub-timeslot of each timeslot of theparticular frequency hopping sequence.
 3. The method as in claim 1,wherein sampling during only the specified sub-timeslot comprisessampling the specified sub-timeslot of only certain timeslots of theparticular frequency hopping sequence.
 4. The method as in claim 1,wherein continuing to sample the particular frequency hopping sequencefor at least one or more additional sub-timeslots of the remainder ofthe timeslot comprises: determining whether the transmission energy isfrom a transmission meant for the communication device; keeping thereceiver on for at least a duration of the transmission in response tothe transmission being meant for the communication device; and turningoff the receiver in response to the transmission not being meant for thecommunication device.
 5. The method as in claim 1, further comprising:selecting operation of the communication device between the first modeand a second mode; and operating the communication device in the secondmode, wherein the second mode comprises: sampling the particularfrequency hopping sequence by the receiver of the communication deviceduring all non-transmitting sub-timeslots of the particular frequencyhopping sequence.
 6. The method as in claim 5, wherein selectingcomprises: selecting the first mode in response to a power reduction atthe communication device.
 7. The method as in claim 1, furthercomprising: informing one or more neighboring devices that thecommunication device is operating according to the first mode.
 8. Themethod as in claim 7, wherein informing comprises: including anindication within a message selected from a group consisting of: anexplicit indication message having the indication; an outgoing messagehaving the indication piggybacked therein; and an outgoingacknowledgement message having the indication piggybacked therein.
 9. Amethod, comprising: operating a communication device according to aparticular frequency hopping sequence in a communication network, theparticular frequency hopping sequence having a plurality of timeslotseach corresponding to a particular frequency within the frequencyhopping sequence, each timeslot divided into a plurality ofsub-timeslots; and determining whether a neighboring communicationdevice is operating in a first mode or a second mode; in response todetermining that the neighboring communication device is operating inthe second mode, transmitting a transmission to the neighboringcommunication device starting at any sub-timeslot of the plurality ofsub-timeslots; and in response to determining that the neighboringcommunication device is operating in the first mode, transmitting thetransmission to the neighboring communication device while ensuring thatthe transmission is actively energized during a particular specifiedsub-timeslot.
 10. The method as in claim 9, wherein the specifiedsub-timeslot is an initial sub-timeslot of a particular timeslot. 11.The method as in claim 9, wherein determining whether the neighboringcommunication device is operating in the first mode or the second modecomprises: assuming that the neighboring communication device isoperating in the second mode as a default mode.
 12. The method as inclaim 11, wherein determining that the neighboring communication deviceis operating in the first mode comprises: transmitting the transmissionto the neighboring communication device starting at any sub-timeslot ofthe plurality of sub-timeslots; determining that no acknowledgement forthe transmission is received from the neighboring communication device;and assuming that the neighboring communication device is operating inthe first mode based on determining that no acknowledgement for thetransmission is received from the neighboring communication device. 13.The method as in claim 12, further comprising: determining that noacknowledgement for the transmission is received from the neighboringcommunication device in response to transmitting the transmission to theneighboring communication device while ensuring that the transmission isactively energized during a particular specified sub-timeslot accordingto the first mode; and in response, declaring the neighboringcommunication device unreachable.
 14. The method as in claim 9, whereindetermining that the neighboring communication device is operating inthe first mode comprises: determining that the neighboring communicationdevice is operating in the first mode in response to receiving anindication within a message received from the neighboring communicationdevice that informs the communication device that the neighboringcommunication device is operating according to the first mode.
 15. Themethod as in claim 9, wherein ensuring that the transmission is activelyenergized during a particular specified sub-timeslot comprises:transmitting one or more contiguous wake-up transmissions prior to thespecified sub-timeslot and during the specified sub-timeslot to accountfor potential clock-drift of the neighboring communication device. 16.An apparatus, comprising: a processor; and a receiver configured tocommunicate in a communication network according to a particularfrequency hopping sequence, the particular frequency hopping sequencehaving a plurality of timeslots each corresponding to a particularfrequency within the frequency hopping sequence, each timeslot dividedinto a plurality of sub-timeslots according to a first mode; wherein thereceiver is configured to operate in a first mode, wherein the receiverand processor when operated in the first mode are operable to: samplethe particular frequency hopping sequence during only a particularspecified sub-timeslot of a timeslot; detect whether a transmissionenergy is detected during the specified sub-timeslot; and in response todetecting that there is no transmission energy during the specifiedsub-timeslot, turn off the receiver for a remainder of the timeslot; andin response to detecting that there is transmission energy during thespecified sub-timeslot, continue to sample the particular frequencyhopping sequence for at least one or more additional sub-timeslots ofthe remainder of the timeslot.
 17. The apparatus as in claim 16, whereinthe receiver and processor, when operated in the first mode to sampleduring only the specified sub-timeslot, are further operable to: samplean initial sub-timeslot of each timeslot of the particular frequencyhopping sequence.
 18. The apparatus as in claim 16, wherein the receiverand processor, when operated in the first mode to continue to sample theparticular frequency hopping sequence for at least one or moreadditional sub-timeslots of the remainder of the timeslot, are furtheroperable to: determine whether the transmission energy is from atransmission meant for the receiver; keep the receiver on for at least aduration of the transmission in response to the transmission being meantfor the receiver; and turn off the receiver in response to thetransmission not being meant for the receiver.
 19. The apparatus as inclaim 16, wherein the receiver and processor are further operable to:select operation of the receiver between the first mode and a secondmode; and wherein the receiver and processor when operated in the secondmode are operable to: sample the particular frequency hopping sequenceby the receiver during all non-transmitting sub-timeslots of theparticular frequency hopping sequence.
 20. The apparatus as in claim 19,wherein the receiver and processor, when operable to select, are furtheroperable to: select the first mode in response to a power reduction atthe apparatus.
 21. An apparatus, comprising: a processor; and atransmitter configured to communicate in a communication networkaccording to a particular frequency hopping sequence, the particularfrequency hopping sequence having a plurality of timeslots eachcorresponding to a particular frequency within the frequency hoppingsequence, each timeslot divided into a plurality of sub-timeslotsaccording to a first mode; wherein the transmitter and processor whenoperated are operable to: determine whether a neighboring communicationdevice is operating in a first mode or a second mode; in response todetermining that the neighboring communication device is operating inthe second mode, transmit a transmission to the neighboringcommunication device starting at any sub-timeslot of the plurality ofsub-timeslots; and in response to determining that the neighboringcommunication device is operating in the first mode, transmit thetransmission to the neighboring communication device while ensuring thatthe transmission is actively energized during a particular specifiedsub-timeslot.
 22. The apparatus as in claim 21, wherein the transmitterand processor, when operable to determine whether the neighboringcommunication device is operating in the first mode or the second mode,are further operable to: assume that the neighboring communicationdevice is operating in the second mode as a default mode.
 23. Theapparatus as in claim 22, wherein the transmitter and processor, whenoperable to determine whether the neighboring communication device isoperating in the first mode or the second mode, are further operable to:transmit the transmission to the neighboring communication devicestarting at any sub-timeslot of the plurality of sub-timeslots;determine that no acknowledgement for the transmission is received fromthe neighboring communication device; and assume that the neighboringcommunication device is operating in the first mode based on determiningthat no acknowledgement for the transmission is received from theneighboring communication device; and declare the neighboringcommunication device unreachable in response to determining that noacknowledgement for the transmission is received from the neighboringcommunication device in response to transmitting the transmission to theneighboring communication device while ensuring that the transmission isactively is energized during a particular specified sub-timeslotaccording to the first mode.
 24. The apparatus as in claim 21, whereinthe transmitter and processor, when operable to determine that theneighboring communication device is operating in the first mode, arefurther operable to: determine that the neighboring communication deviceis operating in the first mode in response to an indication within amessage received from the neighboring communication device that informsthe apparatus that the neighboring communication device is operatingaccording to the first mode.
 25. The apparatus as in claim 21, whereinthe transmitter and processor, when operable to ensure that thetransmission is actively energized during a particular specifiedsub-timeslot, are further operable to: transmit one or more contiguouswake-up transmissions prior to the specified sub-timeslot and during thespecified sub-timeslot to account for potential clock-drift of theneighboring communication device.